Machinery with rotors, such as electrical generators, steam and gas turbines, and large motors, commonly use journal type main bearings due to their high load-carrying capacity. In a journal bearing the rotor shaft is supported on a film of oil in clearance between the journal and the bearing inner surface or babbitt. The oil film is maintained by the rotation of the journal. Oil circulates through the bearing and exits through a drain line, then recirculates via a pump, filter, reservoir, and cooler. Circulation maintains the oil film, cools the bearing, and removes contaminants. If oil pressure is lost, the journal settles onto the babbitt and frictionally overheats, causing extensive damage to the shaft, bearing, and other parts.
In power generation equipment, the rotor may take about an hour to rundown during a non-braked shutdown. Even with braking, a rundown with a loss of bearing lubrication has time to cause serious damage.